[mapserver-dev] Fwd: Re: [OSGeo-Standards] Web Feature Service (WFS) and Axis Order

[Jeff McKenna]

Forwarding to the MapServer-dev list, as it is an interesting discussion...



-------- Forwarded Message --------
Subject: 	Re: [OSGeo-Standards] Web Feature Service (WFS) and Axis Order
Date: 	Wed, 19 Oct 2016 10:21:38 -0600
From: 	Carl Reed <carl.n.reed at gmail.com>
To: 	standards at lists.osgeo.org



Dear OSGeo community -

I have subscribed to the GeoServer list but have not yet been approved 
so I am posting to this list.

As you may know years ago I facilitates a lengthy and at time lively OGC 
discussion on Axis Order and CRS. The result of these discussions was 
OGC member approval of the OGC Axis Order policy and guidance 
(http://www.ogcnetwork.net/axisorder).

Given this background, Esri recently approached me about some 
ambiguities and confusions regarding the history and evolution of how 
axis order was expressed in a Web Feature Service implementation 
instance. After review of various OGC WFS documents and discussions with 
the Editor (Peter Vretanos) I compiled the following information. I will 
be sharing the same information with the OGC Architecture Board and 
staff. Apparently, the CRS axis order issue never really "dies"!!

I bring this information to your attention so that perhaps the GeoServer 
community can update their WFS Axis Order web page content to be 
consistent with the following. Any questions in that regard, please let 
me know. Onto the discussion.

Part of the confusion is probably due to changing OGC policies over the 
last 15 years. Examples are the 2008 Axis Order Guidance policy 
(08-138r5) and later the http uri policy (in TC Policy Directives). 
Anyway, in 2013 the WFS 1.1.1 corrigendum corrected the axis order issue 
to be consistent with the Axis Order policy as well as other OGC standards.

Short synopsis: Before WFS 1.1.1 the only sure thing was that the axis 
order was LON/LAT when the notation "EPSG:4326" was used to define the 
CRS and otherwise was undefined.  For WFS 1.1.1 and later, the OGC Axis 
Order policy was in force which meant and still means that the axis 
order is defined by the CRS being referenced, such as lat/lon for EPSG 4326.

The longer answers is:

How any WFS instance interprets axis order depends on the version of the 
WFS standard implemented and the notation being used to specify the CRS.

Based on the WMS 1.0 model, WFS 1.0.0 used the notation EPSG:XXXX for 
specifying CRSs.  The implication was (although it was never explicitly 
stated) that the WFS would do what WMS did and what the WMS did was to 
say that for EPSG:4326 the axis order was LON/LAT. This was actually 
also consistent with GML 2.1.1. For example:

<gml:Box srsName="http://www.opengis.net/gml/srs/epsg.xml#4326 
<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_gml_srs_epsg.xml-234326&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=cz2ENba-ejcGrGosCxt7VCq5sYUHu7GlMCLq3G1jO2M&e=>">
           <gml:coordinates>-180.0,-90.0 180.0,90.0</gml:coordinates>
</gml:Box>

The WMS 1.0 standard did not say anything about any other EPSG code and 
so other CRS behavior was undefined.  As a result, most WFS 1.0.0 
implementations stuck with LON/LAT regardless of the EPSG code being used.

WFS 1.1.0 specified several notations for defining the CRS: EPSG:XXXX, 
http://www.opengis.net/gml/src/epsg.xml#XXXX 
<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_gml_src_epsg.xml-23XXXX&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=vcSZ-tEvudUXHYfE6HX2WUCkg5qjM9X5ULCLBSADUkw&e=>, 
urn:EPSG:geographicCRS:XXXX.  The EPSG:XXXX notation was maintained for 
backward compatibility with WFS 1.1. As such, a WFS implementation 
instance would behave as it did in WFS 1.0.0 (i.e. LON/LAT for 
EPSG:4326, undefined otherwise).  The other two notations were meant to 
follow the then being developed OGC Axis Order policy which eventually 
provided clear policy guidance on how to express and document axis 
order. The recommended option is to use the axis order as defined in the 
EPSG CRS definition.  However, at the time WFS 1.1 was being processed 
the Axis Order policy and notation were still in flux. Ultimately the 
URL and URN notations specified in the WFS 1.1 standard were invalid. 
This problem was, however, addressed in the WFS 1.1.1 corrigendum. 
Unfortunately, for some reason that corrigendum was never published 
thereby increasing the WFS axis order confusion. This oversight is being 
addressed by OGC staff.

Which brings us to WFS 1.1.1 and 2.0.X.  In these versions, the same 
Axis Order policy is specified: Do what the EPSG CRS definition says. 
Also, at the same time the srsName expression changed to the http uri 
notation (i.e. http://www.opengis.net/dev/crs/epsg/0/xxxx 
<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.opengis.net_dev_crs_epsg_0_xxxx&d=DQMFaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=LCXhcrMdvdcRNz8NUgOEV1cYOakNFpC9sSTsSszC0Kc&m=HY5mDJJy4-L8dvfBgWuyrG9fxDiNEyFRZNZCetAE2kY&s=otM5bDnCQOb9DY-U_pSooY0tHUmXzTpcwfIOXZvwxyk&e=>). 
This is because in 2010 the OGC policy changed regarding how to identify 
resources. The OGC moved from urn's to using URLs as identifiers. Even 
so, the initial version of WFS 2.0 used the URN notation 
(urn:ogc:def:crs:EPSG:<ver>:XXXX). This changed in WFS 2.0.2 
(corrigendum). All of the urn references were changed to URLs with notes 
on what the older "urn" strings were.

Which brings us to the GeoServer documentation. That documentation is 
not quite right and should be corrected to reflect reality and what is 
actually in the various versions of the WFS standards. Further, the 
GeoServer documentation mentions the use of "urn:x-". I am not sure 
where that is coming from as none of the WFS standards from WFS 1.0 and 
later use that notation.

Regards

Carl
-- 
Carl Reed, PhD
Carl Reed and Associates

Mobile: 970-402-0284

"When the power of love overcomes the love of power the world will know 
peace." Jimi Hendrix